04 Mr. Merrifield's incidental ohnervafionn 



second 5, it looks as if the frosting in this case had the 

 specific efi'ect of caushig the subsequent partial retarda- 

 tion and great irregularity in emergence, an irregularity 

 represented by a range of from 19 to 50 days, though 

 the facts as to the other 2 lots of 5 each which emerged 

 respectively in 11 — 13 days and 6 — 8 days seem to show 

 that the earliness of the date at which the forcing began 

 had something to do with this. All are of the spring 

 type, and there is no marked difference between those 

 which have been frosted and the rest. I have reason to 

 think the unfrosted pupfe were never at a lower tempera- 

 ture than 40% which therefore, if and so far as lowness 

 of temperature is a cause of this dark colouring, appears 

 to be low enough for the purpose in illustraria. 



Effects of forcing in different cases. — It seems to be an 

 interesting question what are the causes which determine 

 whether the pup^e of a double-brooded moth shall fall 

 into the category of those that will emerge in spring, or 

 into the category of those that will emerge in summer. 

 There is of course a very real distinction between the 

 two categories, one obstinately resisting for months a 

 high temperature to which the other succumbs in a few 

 daj^s ; and part of a brood will range itself in one 

 category, and the rest of it, though subjected to exactly 

 the same conditions, in the other. The different 

 behaviour, with me, of the two sj^ecies will be noticed, 

 illunaria yielding several forced broods in succession, 

 while illustraria gave one and part only of the next. 

 The case of illunaria ^iroves, if proof were needed, that it 

 is not a case of alternative succession of the two different 

 forms, while the instance given of the forced illustraria 

 splitting into two shows that some other cause than a 

 difference of temperature must be concerned in occa- 

 sioning it. Though temperature appears to have so 

 little effect on a pupa that is not ripe for it, the effect of 

 it on a pupa which is ripe for its operation seems imme- 

 diate and tolerably regular, both with the spring and the 

 summer emergence. [I give a summary of the effect of 

 forcing on the pupae of some considerable broods : — 



